1. Field of the Invention
The present invention pertains to a method and apparatus for processing a video signal, and more particularly to improving the effects of phase modulation of the color burst component of the video signal for video copy protection).
2. Description of the Prior Art
Various copy protection techniques have been developed to modify a video signal so as to prevent copying or reduce the entertainment value of a copied videocassette (effectiveness) while the same signal produces a display on a television receiver or monitor with a minimum of or no visible artifacts.
Video copy protection is defined as a system whereby a copy protected video signal is viewable with a minimum of or no visible artifacts whereby the playback of a recording of such a signal is not possible or produces a signal that has significantly degraded entertainment value. Copy protection is to be differentiated from video scrambling. Video scrambling means that the video signal is not viewable. A scrambled signal may be recordable, but unless it has been descrambled, the playback of such a recording is still unviewable.
A well known copy protection scheme for video signals include is that disclosed in U.S. Pat. No. 4,631,603 ('603), by John O. Ryan, issued on Dec. 23, 1986 and assigned to Macrovision, incorporated by reference. The '603 patent is directed to modifying an analog video signal to inhibit making of acceptable video recordings therefrom. This discloses adding a plurality of pulse pairs to the otherwise unused lines of a video signal vertical blanking interval, each pulse pair being a negative-going pulse followed closely by a positive-going pulse. The effect is to confuse AGC (automatic gain control circuitry) of a VCR (video cassette recorder) recording such a signal, so that the recorded signal is unviewable due to the presence of an excessively dark picture when the recorded signal is played back.
Another well known copy protection is that disclosed in U.S. Pat. No. 4,577,216 ('216), “Method and Apparatus For Processing a Video Signal,” John O. Ryan, issued Mar. 18, 1986 and incorporated by reference, discloses modifying a color video signal to inhibit the making of acceptable video recordings thereof. A conventional television receiver produces a normal color picture from the modified signal. However, the resultant color picture from a subsequent video tape recording shows variations in the color fidelity that appear as bands or stripes of color error. Colloquially the modifications are called the “Colorstripe™ system” or the “Colorstripe™ process”. Commercial embodiments of the teachings of this patent have typically limited the number of video lines per field having the induced color error or color stripes.
The teachings of the '603 patent are useable in analog video cassette duplication and various digital transmission and recorder playback system such as DVD, DVCR and satellite services utilizing a digital set top decoder. The teachings of the '603 patent depend upon the actions of the AGC of a recorder. The recorders used in the video cassette duplication facilities are specially modified to operate without depending upon this AGC action and can thus record the copy protected signal. The Colorstripe™ system depends upon the color time base recording system of a video cassette recorder. It is not economically possible to modify the duplicating video cassette recorders to record a signal with the teachings of the '216 patent. Thus the Colorstripe™ system is used primarily in transmission systems; on the output of DVD recorders and playback machines; and on the output of DVCR machines. A fuller discussion on how the Colorstripe™ system is incorporated in these systems is discussed below.
Color video signals (both in the NTSC and PAL TV systems) include what is called a color burst. The color stripe system modifies the color burst. The suppression of the color subcarrier signal at the TV transmitter requires that the color TV receiver include an oscillator (in NTSC a 3.58 MHz oscillator) (in PAL a 4.43 MHz oscillator) which is used during demodulation to reinsert the continuous color subcarrier signal and restore the color signal to its original form. Both the frequency and phase of this reinserted subcarrier signal are critical for color reproduction. Therefore, it is necessary to synchronize the color TV receiver's local 3.58 MHz or 4.43 MHz oscillator so that its frequency and phase are in step with the subcarrier signal at the transmitter.
This synchronization is accomplished by transmitting a small sample of the transmitter's 3.58 MHz or 4.43 MHz subcarrier signal during the back porch interval of the horizontal blanking pulse. FIG. 1A shows one horizontal blanking interval for an NTSC color TV signal. FIGS. 1B and 1C show the details the color burst on two lines of the video signal. The phase of the color burst on successive lines in the NTSC are 180 degrees out of phase with each other. The horizontal sync pulse, the front porch and blanking interval duration are essentially the same as that for black and white TV. However, during color TV transmission (both broadcast and cable) 8 to 10 cycles of the 3.58 MHz (in NTSC) subcarrier that is to be used as the color sync signal are superimposed on the back porch. This color sync signal is referred to as the “color burst” or “burst”. The color burst peak-to-peak amplitude (40 IRE for NTSC TV as shown) is generally the same amplitude as the horizontal sync pulse.
FIG. 1B shows an expanded view of a part of the waveform of FIG. 1A including the actual color burst cycles. During the color TV blanking intervals, such a color burst is transmitted following each horizontal sync pulse. Similar characteristics for the horizontal blanking interval and color burst are present in a PAL signal. The differences between PAL and NTSC are discussed more fully below.
The phase relationship of the color burst and the color components of an NTSC signal are shown in FIG. 1D. The NTSC color system operates on a quadrature modulation system based upon an R-Y and B-Y or an I and Q system. For ease of discussion, we will discuss the R-Y and B-Y system. As can be seen on FIG. 1D the R-Y axis is the vertical axis and the B-Y axis is the horizontal axis. The Color Burst signal has been specified to be on the B-Y axis and is at the 180 degree point relative to a 0 degree point as shown on FIG. 1D. The color modulation demodulation process depends upon this phase relationship between the various color components shown in the vector diagram of FIG. 1D and the reference subcarrier represented by the color burst signal. The color stripe processes described in the '216 patent and the material below represent a modification of this phase relationship that creates a copy protected signal that has an effectiveness to produce a recording of the signal that has lost its entertainment value while the copy protected signal is displayed without artifacts by a TV receiver or monitor (playability).
The phase relationship of the color burst and the color components of an unmodified PAL signal are shown in FIG. 2C. The PAL color system like its NTSC counterpart operates on a quadrature modulation system based upon an U axis and V axis. As can be seen on FIG. 2C the V axis is the vertical axis and the U axis is the horizontal axis. One of the key differences between the NTSC color system and the PAL TV system is the vector location of the color burst. The PAL Color Burst signal has been specified to be at +/−45 degrees from the −U Axis relative to a 0 degree point as shown on FIG. 2C. On an alternating line basis the V signal switches 180 degrees in phase. The color burst of each of these switches in synchronization. On the lines with a +V signal, the color burst is at +45 degrees relative to the U axis. On the lines with a −V signal, the color burst is at −45 degrees relative to the U axis. The color modulation demodulation process depends upon this phase relationship between the various color components shown in the vector diagram of FIG. 2C and the reference subcarrier represented by the color burst signal. The so called swinging burst is used to generate a PAL ID pulse within the demodulation process to appropriately switch the demodulation process to respond to the line by line change in the V portion of the signal. Due to the unique characteristics of the PAL TV signal there is a need for an improvement on the color stripe processes described in the '216 patent to improve to create a copy protected signal that has an effectiveness to produce a recording of the signal that has lost its entertainment value while the copy protected signal and to improved the display of the signal without artifacts by a TV receiver or monitor. For further details on the PAL Color TV system see “Color Television” by Geoffrey Hutson, Peter Shepherd, and James Brice, published by McGraw Hill Book Company, (UK) Limited, Maidenhead, Berkshire, England.
In the embodiments of the color stripe process, no color burst phase (stripe) modification appears in the video lines that have a color burst signal during the vertical blanking interval. These are lines 10 to 21 in an NTSC signal and corresponding lines in a PAL signal. The purpose of keeping these lines modified is to improve the playability of the modified signal. Since these lines are not visible on the playback of a recording there is no improved effectiveness to be achieved by modifying these lines. The previous commercial embodiments of the color stripe modifications (modulation of the color burst phase) have occurred in bands of four to five video lines of the viewable TV field followed by bands of eight to ten video lines without the color stripe modulation. The location of the bands is fixed (“stationary”) field-to-field. This color stripe process has been found to be quite effective for cable television, especially when combined with the teachings of the '603 patent discussed above.
In NTSC TV, the start of color burst is defined by the zero-crossing (positive or negative slope) that precedes the first half cycle of subcarrier (color burst) that is 50% or greater of the color burst amplitude. It is to be understood that the color stripe process shifts the phase of the color burst cycles relative to their nominal (correct) position which is shown in FIG. 1B. The phase shifted color burst is shown in FIG. 1D. The amount of phase shift shown in FIG. 1C may be as great as 180° (the maximum possible).
Further, the amount of phase shift in the color stripe process can vary from e.g. 20° to 180°; the more phase shift, the greater the visual effect in terms of color shift. In a color stripe process for PAL TV, a somewhat greater phase shift (e.g. 40° to 180°) is used to be effective.
In any copy protection system there is a need for a proper balance between the effectiveness of the copy protected signal in the making of a deteriorated copy versus the need for no visible effects on the playability of the copy protected signal. However, certain television sets may produce slight playability problems when displaying a signal with embodiments of the '216 patent. In particular, the visibility of the color stripe on a television receiver has been found to be particularly noticeable in certain “picture in picture” (“p-i-p”) portions of the TV display. These systems use analog to digital conversion and digital to analog conversion techniques to accomplish the “p-i-p” feature that may be sensitive to the phase errors of the '216 embodiment. Therefore, it is an object of this invention to provide an improved method and apparatus for modifying a color video signal so that a conventional television receiver produces a normal color picture including the picture in picture portion from the modified signal, whereas a videotape recording made from the modified signal exhibits annoying color interference, thus discouraging or inhibiting videotaping of the signal.
Other variations of the Colorstripe™ process are also possible.